Block的优点

关于Block的语法我就不讲了，请看这篇文章。

刚开始学习block的时候，觉得比较奇怪，block到底有什么用？它和函数有什么区别？经过一番学习，主要是这篇文章，发现block有以下几个地方最有用：

1.简化的回调过程，不用再实现并调用某个函数，如Notification的方法，以前是这么写：

[[NSNotificationCenter defaultCenter] addObserver:self selector:@selector(keyboardWillShow:) name:UIKeyboardWillShowNotification object:nil];
- (void)keyboardWillShow:(NSNotification *)notification {   
// Notification-handling code goes here.   
}

现在可以这么写：

[[NSNotificationCenter defaultCenter] addObserverForName:UIKeyboardWillShowNotification 
    object:nil queue:[NSOperationQueue mainQueue] usingBlock:^(NSNotification *notif)

或者UIView动画结束时的操作：

用

animateWithDuration:delay:options:animations:completion:

代替

setAnimationDidStopSelector:(SEL)selector

2.用作枚举类型遍历的方法，即相当于

for (id item in collection) {   
    // Code to operate on each item in turn.   
}

如

NSString *area = @"Europe";   
NSArray *timeZoneNames = [NSTimeZone knownTimeZoneNames];   
NSMutableArray *areaArray = [NSMutableArray arrayWithCapacity:1];   
NSIndexSet *areaIndexes = [timeZoneNames indexesOfObjectsWithOptions:NSEnumerationConcurrent   passingTest:^(id obj, NSUInteger idx, BOOL *stop) {   NSString *tmpStr = (NSString *)obj;   
return [tmpStr hasPrefix:area];   
}];   
NSArray *tmpArray = [timeZoneNames objectsAtIndexes:areaIndexes];   
[tmpArray enumerateObjectsWithOptions:NSEnumerationConcurrent|NSEnumerationReverse   
usingBlock:^(id obj, NSUInteger idx, BOOL *stop) {   
[areaArray addObject:[obj substringFromIndex:[area length]+1]];   
}];   
NSLog(@"Cities in %@ time zone:%@", area, areaArray); 

3.排序

NSComparator finderSort = ^(id string1, id string2) {   
        NSRange string1Range = NSMakeRange(0, [string1 length]);   
        return [string1 compare:string2 options:comparisonOptions range:string1Range locale:currentLocale];   
};   
NSLog(@"finderSort: %@", [stringsArray sortedArrayUsingComparator:finderSort]);  

4.并发，在GCD和NSOperationQueue中使用。这个还没有仔细研究。